Various techniques for detecting step-out of an electric motor have been conventionally developed. Among the documents exemplifying such techniques are Japanese Patent Application Laid-Open No. 2010-051151, Japanese Patent Application Laid-Open No. 2010-252503, and Japanese Patent Application Laid-Open No. 2008-92787.
Japanese Patent Application Laid-Open No. 2010-051151 discloses a technique for determining step-out from a deviation of a d-axis current and a q-axis command voltage.
Japanese Patent Application Laid-Open No. 2010-252503 discloses a technique for detecting step-out by comparison between a model voltage and a voltage command.
Japanese Patent Application Laid-Open No. 2008-92787 discloses a technique for determining step-out if a magnetic flux obtained from a current and a rotation speed command is not greater than a threshold.